articulating arm help

[Madison]

This isn't quite an elegant solution, but you might consider adding a window motor to the system geared/matched to the RPM at one of your rotating joints in addition to the BB motors you're using to rotate the arm. The window motor's worm gear could help to lessen the need for the BB motors to operate at/near stall to hold your arm in place.

[HarveyAce]

but i thought that the window motors couldn't backdrive due to the worm gearing inside?

[JamesBrown]

Originally Posted by HarveyAce

but i thought that the window motors couldn't backdrive due to the worm gearing inside?

I think you are confusing backdrive and drive backwards. If you reverse the voltage to the motor, it will drive in the other direction just like any of the other motors.

What people mean when they say you can not backdrive the window motor is that you can't physically turn the motor by turning the shaft (the problem the OP is having).

[HarveyAce]

OH! thats what that meant. my bad... that opens up my options again! thanks! at about how long of straight running will the window motors thermal lock kick in?

[justibm]

Originally Posted by JamesBrown

I think you are confusing backdrive and drive backwards. If you reverse the voltage to the motor, it will drive in the other direction just like any of the other motors.

What people mean when they say you can not backdrive the window motor is that you can't physically turn the motor by turning the shaft (the problem the OP is having).

If you can't turn the window motor with it dead, what would be the best way to move it back to original position when the match is over?

[Alan Anderson]

Originally Posted by justibm

If you can't turn the window motor with it dead, what would be the best way to move it back to original position when the match is over?

Add a mechanical clutch to disengage the mechanism from the motor when desired. This can be done in many ways; I will not limit your creativity by suggesting one.

[Madison]

Originally Posted by HarveyAce

OH! thats what that meant. my bad... that opens up my options again! thanks! at about how long of straight running will the window motors thermal lock kick in?

It depends on how they're loaded. The more current they're drawing, the warmer they'll get and the faster they'll trip their internal thermal breaker.

I would recommend against using ONLY window motors to drive your arm. Once geared appropriately to move an arm, you'll find that they're quite slow.

[Joe Ross]

Originally Posted by Alan Anderson

If you intend to use the motor alone to hold the arm up, without any mechanical counterweight or spring-loading, be cautious. You'll be running the motor at or near stall a lot of the time. A CIM can handle that for a while, but it will get very warm and you might want to consider a way to cool it off during or between matches.

A Fisher-Price or Banebots motor will not handle that kind of abuse well at all. They rely on an internal fan to keep their windings cool, and if they're not turning, neither is the fan. You'll melt something in short order, followed by smoke. Nearby veterans will sniff the air and nod at the familiar smell of a fried motor.

We've successfully run fisher price and banebots motors stalled at between 5-10% in order to keep mechanisms in place. The trick is to design the mechanical system such that only that little power is needed. If you do need much more then that, your motors will burn.

[MrForbes]

I agree with Joe, you can make a little motor live when driving an arm, but you need to be very careful to make sure it's not going to need to do much work to move the arm, and very little at all to hold it still. We did this in 2007 and it worked fine, we used a gas spring to support the weight of the arm itself, so the motor had very little work to do.

This year we're going to try out a window motor or two for the arm. We will again counter the weight of the arm with a springy type thing. We are also going to add some reduction between the motor and arm, both to slow the arm movement (1/4 second for full arm travel is a bit too fast!) and to reduce the load on the motor. And we have a plan for a nifty way to do this but still allow for that clutch thing that Alan was hinting at. The drive has a built in clutching ability, and it's cheap and locally available. And we've used it before for a different application...we know the slipping part works, which is good because we seem to be unable to get limit switches working

[pfreivald]

No love here for the screw drive?

[MrForbes]

not really....kinda hard to use effectively.

If you have some ideas of how to make it work well, I'm sure others would like to see them.

[pfreivald]

We're using four (4) on our robot this year...

We used one on our robot last year, and iirc were tied for the most consistent hanger for the first three weeks of competition.

I have a lot of love for the screw drive, but perhaps it's because there are several linear motion actuator companies in the area, and that's pretty much what all of them make...

[artdutra04]

Originally Posted by quinxorin

The mechanical solution, guaranteed to work, would be to replace your CIMple box with a wormgear. It cannot be backdriven. This is the same type of gearbox as comes preinstalled on the window motors.

Worm gears can backdrive, although depending on the angle of the teeth on the worm, the gear material, and whether it was lubricated or not, the torque required to backdrive the worm gear may exceed what the teeth of the gears can withstand. Thus, for a limited range of torque, worm gears can be considered non-backdrivable.